What is the purpose of inclusion and exclusion criteria in study design?

• A. Exclude All Extremes
• B. Define Population; Improve Validity; Reduce Confounding; Enhance Generalizability
• C. Define Population But Increase Bias
• D. Increase Sample Size

Answer: (B)

Inclusion and exclusion criteria define exactly who can participate in a study. This sets the target population and ensures the study focuses on a comparable group, which helps the results be interpretable and meaningful.

By specifying who is eligible, you reduce variation from people with different characteristics that could confound the relationship between exposure and outcome. This strengthens internal validity because differences in outcomes are more likely due to the exposure or intervention rather than other factors. At the same time, clearly delineating the population helps readers judge to whom the findings apply, supporting generalizability to similar groups.

Choosing criteria that are too broad can introduce heterogeneity and bias, while overly strict criteria can limit applicability. The goal is a clear, appropriate definition of who is studied to balance validity with relevance.

Other options misstate the purpose: excluding extremes isn’t the primary aim, criteria should not be viewed as increasing bias, and inclusion/exclusion don’t aim to raise the sample size.